For the next generation long-distance and mass communication system implementation, studies have been made on a technique for providing pre-distortion information to a modulator input signal by digital signal processing at a sender side, or generating a desired multilevel signal by a software defined radio technology (for the latter, see Non-Patent Document 1 listed below).
In a digital signal processing (DSP) based system, transmission signal characteristics can be optimized by performing a process to compensate for signal degradation occurring during transmission, on modulator input signals. However, an optical modulator itself used in a modulation part is generally a conventional modulator (for example, an LN modulator of Mach-Zehnder type) with the power of an optical output changing periodically depending on a drive voltage. Accordingly, degradation of a transmission signal due to drift is still a technical issue to be solved. To solve this problem, automatic bias control (ABC) is employed.
As a modulation control technology corresponding to a modulation scheme such as differential quadrature phase-shift keying (DQPSK), a method for compensating for fluctuation in operating points is known (see, for example, Patent Document 1 listed below). In this method, a low-frequency signal is superimposed on a drive signal being input to an optical modulator, a portion of the optical signal output from the optical modulator is monitored to detect the low-frequency component, and fluctuation in the operating point is compensated for based upon the detected low-frequency component.
FIG. 1 illustrates a configuration of a conventional 16-QAM optical modulation device 1000. Four-level electrical drive signals are generated by a digital signal processor 1001 and digital-analog converters (DAC) 1002 and 1003, and input to the 16-QAM optical modulation device 1000. Carrier light generated at a light source 1007 is modulated by the four-level electrical drive signals. To be more precise, the four-level drive signals are supplied to the I-arm 10081 and the Q-arm 1008Q of an LN modulator 1008, respectively, to drive the LN modulator 1008 at an amplitude of 2×Vπ of the four-level drive signal. A π/2 phase shifter 1009 gives a π/2 phase difference between the optical signals from the I-arm 10081 and the Q-arm 1008Q. The optical signals with the phase difference are combined and output as a 16-QAM optical signal.
FIG. 2A and FIG. 2B are diagrams illustrating a method for controlling a bias voltage applied to a LN modulation device 1100 operated at a driving voltage of 2×Vπ. This method is applicable to such a modulation scheme that drive voltage/light intensity characteristics of a optical modulator change periodically between peak and trough and modulation is carried out using an electrical signal with an amplitude of 2×Vπ (as illustrated in FIG. 2B). Such modulation scheme includes CSRZ modulation, optical duo-binary modulation, differential phase-shift keying (DPSK), DQPSK, etc.
In FIG. 2A, electrical signals of amplitude Vπ with the sign reversed are inputs through drive circuits 1104 and 1105 to the electrodes of the arm 1108a and the arm 1108b, respectively, of the optical modulator (an LN modulator). The optical modulator 1108 is push-pull driven at the driving amplitude of 2×Vπ. A low-frequency signal f0 generated by a low-frequency generator 1115 is applied to the optical modulator 1108 together with a bias voltage. A portion of the output of the optical modulator 1108 is detected at a photodiode 1111 and converted to an electrical signal. A phase comparator 1112 extracts a low-frequency component from the electrical signal through phase comparison with the low-frequency signal f0 and supplies the detected low-frequency component to the bias supply circuit 1113. The bias supply circuit 1113 controls the bias voltage such that the low-frequency component becomes about zero. In this manner, a change in the superimposed low-frequency signal component is detected by synchronous detection to carry out feedback control to bring the bias application to the optimum state.
As illustrated in FIG. 2B, when bias application is in the optimum state “a”, namely, when the bias voltage is at a trough of the drive voltage/light intensity characteristics, low-frequency component f0 is not produced in the optical output signal. If the bias voltage deviates from the optimum state to the state “b” or the state “c”, the f0 component is produced. Since the phase of the f0 component inverts according to the direction of deviation from the optimum bias point, the direction of deviation can be known by monitoring the phase of the detected f0 component. Then bias is applied so as to correct the deviation to the optimum bias point “a”.
However, this method may not be applicable to higher order multilevel modulation formats, such as 16QAM, 64QAM, etc, as described below.
Also, some other bias control techniques are known. In order to maintain behavior at arbitrary operating points other than the maximum and minimum points, inspection light, different from the signal light, is superimposed onto an optical signal being input to a modulator, and the bias voltage applied to the modulator is controlled based upon the modulation state of the inspection light without using a low-frequency signal (see, for example, Patent Document 2 listed below). Another known technique is monitoring a portion of the optical signal output from a modulator to detect a continuing frequency component superimposed onto the signal light and controlling a bias voltage applied to a phase shifter so as to minimize the detected continuing frequency component (see, for example, Patent Document 3 listed below).